Recently, attention is paid to a technology for generating a sequence of features of a signal and then generating the signal based on the sequence of features generated. Such a technology is used to generate a signal (speech signal, music signal, acoustic signal, image signal, video signal, etc.). The signal includes ordinal elements such as time series elements and coordinate series elements. Adjacent ordinal elements have a certain relationship (e.g., continuity) between them.
For example, in the field of speech synthesis for analyzing text and generating synthetic speech (synthetic voice) based on phonetical information represented by the text, the speech is synthesized by generating features (pitch frequencies (heights of voice), phonemic durations (lengths of sound), spectrums, cepstrums, etc.) in a time line by use of the HMM (Hidden Markov Model) widely spread in the field of speech recognition. Also in the field of music synthesis, a music signal is generated similarly by generating features (fundamental frequencies, durations, spectrums, etc. of the music signal) in a time line by use of the HMM. Further, also in the field of image synthesis, an image is generated by generating a sequence of features of DCT (Discrete Cosine Transform) coefficients based on statistics extracted from a large number of images.
Methods usable in the field of speech synthesis for generating a feature sequence have been described, for example, in Non-patent Literatures 1-3. The methods of the Non-patent Literatures 1-3 previously store mean parameters and variance parameters of the HMM. Upon execution of the speech synthesis, the feature sequence is generated by acquiring a mean parameter and a variance parameter for each state in the HMM based on the result of text analysis.
For example, Patent Literature 1 describes that a data output device for outputting time-line data calculates data for times (time points) at prescribed intervals (outputted by a state transition probability model) based on estimated state probabilities and typical data outputted by states and outputs the calculated data as the time-line data. The Patent Literature 1 also describes that the property of a vector time series (sequence) can be changed by adjusting the variance parameter of the transition probability.